Transport properties of diluted magnetic semiconductors: Dynamical mean field theory and Boltzmann theory

The transport properties of diluted magnetic semiconductors (DMS) are calculated using dynamical mean field theory (DMFT) and Boltzmann transport theory. Within DMFT we study the density of states and the dc-resistivity, which are strongly parameter dependent such as temperature, doping, density of the carriers, and the strength of the carrier-local impurity spin exchange coupling. Characteristic qualitative features are found distinguishing weak, intermediate, and strong carrier-spin coupling and allowing quantitative determination of important parameters defining the underlying ferromagnetic mechanism. We find that spin-disorder scattering, formation of bound state, and the population of the minority spin band are all operational in DMFT in different parameter range. We also develop a complementary Boltzmann transport theory for scattering by screened ionized impurities. The difference in the screening properties between paramagnetic ($T>T_c$) and ferromagnetic ($T<T_c$) states gives rise to the temperature dependence (increase or decrease) of resistivity, depending on the carrier density, as the system goes from the paramagnetic phase to the ferromagnetic phase. The metallic behavior below $T_c$ for optimally doped DMS samples can be explained in the Boltzmann theory by temperature dependent screening and thermal change of carrier spin polarization.Comment: 15 pages, 15 figure

Schwoebel barriers on Si(111) steps and kinks

Motivated by our previous work using the Stillinger-Weber potential, which shows that the [$\overline{2}11$] step on 1$\times$1 reconstructed Si(111) has a Schwoebel barrier of 0.61$\pm$0.07 eV, we calculate here the same barrier corresponding to two types of kinks on this step - one with rebonding between upper and lower terrace atoms (type B) and the other without (type A). From the binding energy of an adatom, without additional relaxation of other atoms, we find that the Schwoebel barrier must be less than 0.39 eV (0.62 eV) for the kink of type A (type B). From the true adatom binding energy we determine the Schwoebel barrier to be 0.15$\pm$0.07eV (0.50$\pm$0.07 eV). The reduction of the Schwoebel barrier due to the presence of rebonding along the step edge or kink site is argued to be a robust feature. However, as the true binding energy plots show discontinuities due to significant movement of atoms at the kink site, we speculate on the possibility of multi-atom processes having smaller Schwoebel barriers.Comment: Manuscript in revtex twocolumn format (7pgs - which includes 14 postscript files). Submitted to the The Journal of Vacuum Science and Technology (Proceedings of the Physics and Chemistry of Semi- conductor Interfaces - 23 (1996)

Vibrations of Circular Cylinders of a Perfectly Conducting Elastic Material

The problems of radial vibrations of a long circular solid cylinder with a transverse magnetic field and rotary vibrations of a hollow cylinder with radial magnetic field are solved. The results of the case of an infinite medium with a cylinderical cavity are given. The frequency equation in each case, is solved in particular cases approximately

Correlation induced phonon softening in low density coupled bilayer systems

We predict a possible phonon softening instability in strongly correlated coupled semiconductor bilayer systems. By studying the plasmon-phonon coupling in coupled bilayer structures, we find that the renormalized acoustic phonon frequency may be softened at a finite wave vector due to many-body local field corrections, particularly in low density systems where correlation effects are strong. We discuss experimental possibilities to search for this predicted phonon softening phenomenon.Comment: 4 pages with 2 figure